Alexander III of Macedon, also known as Alexander the Great (356-323 BC) was the king of Macedon from 336 BC - 323 BC.The son of Philip II of Macedon, Alexander embarked on a military campaign against Persia, conquering a large swathe of southern Asia by the end of his death, stopping on the borders of India.
In the OTL, Alexander was tutored by the philosopher Aristotle until he was 16. This occurs in this timeline too, however Aristotle indoctrinates the heir to the throne with his interpretation of Republic. However, although the philosopher has different views toward children, slaves and women, he is still hostile toward outsiders, deeming them barbarians. Aristotle also encourages Alexander to conquer the Persian Empire, viewing it as a threat to his work and an enemy to be destroyed.
The reign of King Alexander
Upon the mysterious death of his father, Alexander takes control of Macedon and begins reorganising it. With his fathers reforms already in place, such as phalanx pikemen rather than hoplites and increased use of cavalry, Alexander conquers Greece and much of the Balkans by 334 BC and then crosses the Hellespont. Just as skilled a general in the OTL, Alexander conquers the Persian Empire by 329 BC.
However, rather than trying to appease Persians through dressing like one and appointing Persian officials into the government, Alexander remains firmly Hellenic. However he marries Roxanne in an attempt to start an interracial dynasty of philosopher-kings, interested in real knowledge rather than just the basics. He adds that any Persian who tries to undermine his position can go the way of pretender King Artaxerxes V (Bessus). This results in an even greater depletion of Macedonian manpower in Greece than in the OTL, but this was countered by Republic demanding for a dedicated military class to keep the peace and wage war. This ensures that Greece is less open to later Roman attack despite the increased manpower drains.
In 327/326 BC, Alexander insisted on going on to India before the Kallipolis (the term used for the state laid out in Republic) is ready, resulting in mutiny and the army turning back. He had planned to reach the mythical ocean around the world and had defeated every Indian army sent against him. However the strength of King Porus finally convinces Alexander's army to stop, not wanting to go any further. Recognising the desire to see their homeland again, Alexander agrees to turn back. On the way, he executes several administrators he left behind over corruption, but rewards all soldiers and allows the old and wounded to return to Macedon. He himself died just off Babylon.
As the first philarchon (philosopher king) of a Kallipolis, Alexander laid the groundwork for the future Kallipolic Confederation. His successors, the Diadochi, fought briefly as nobody knew who "the strongest" was, so nobody knew who should succeed Alexander. Antigonus, Demetrius, Perdiccas, Seleucus, Ptolemy, Lysimachus, Eumenes and Antipater each divided their branches of the Macedonian Empire between them and took charge of a specific area. Also, the idea of the Kallipolis also spread to other Greek colonies such as Massilia (Marseilles) and Magna Graecia (southern Italy). From the south of modern France to India, not only was it possible to only need to speak Greek, but one system of government was used. The nature of the distribution also turned Republic into an almost religious document, with Platonism achieving a status equivalent to Confucianism. This similarity would eventually lead to a war with China. Also, India adopted Republic despite not being defeated as it still held a great appeal to them. If not for Alexander, this may not have happened until much later.
An incredibly important part of Alexander's legacy is the founding of the Egyptian city of Alexandria, where a great Library was established. This Library proved useful in the KC's scientific ventures over the next few millennia.